This invention relates to pin-type, SQW (Single Quantum Well) type and MQW (Multiple Quantum Well) type semiconductor lasers.
It has been known since the 1960's that a semiconductor laser can be made by utilizing the radiative recombination of electrons and holes at a p-n junction. However, it was only about a decade ago that CW lasing became possible in practice. At that time the necessity of having a specific structure for preventing holes and electrons injected from a p-n region from undergoing nonradiative recombination before radiative recombination has not been realized. Moreover, even if the concept was known, techniques for accomplishing such were inmature.
A new technique developed about 15 years ago is called the "double hetero structure". It involves a disposition of a potential barrier for preventing the electrons injected into a p region and the holes injected into an n region from separating a great distance from a junction depletion layer.
The injected electrons and holes must be recombined within a period as short as possible. Therefore, it is obviously most effective to increase the overlap of wave functions of the electrons and holes.
One method to achieve such overlap is to spatially localize the electrons and holes, or the excitons formed by a combination of them.
In conventional semiconductor lasers, a lightly doped layer or an undoped layer which is referred to as an "active layer" and is about 0.1 .mu.m-thick plays a principal role in the recombination. FIG. 1 of the accompanying drawings conceptually shows an energy diagram of the conventional semiconductor lasers. Reference numeral 13 represents the active layer.
In the drawing, reference numeral 11 represents an n-type semiconductor layer; 12 is a barrier layer of an n-type semiconductor; 14 is a barrier layer of a p-type semiconductor; and 15 is a p-type semiconductor layer. The active layer 13 is interposed between the barrier layers 12 and 14 each having a greater band gap than the active layer 13. Reference numeral 10 represents the Fermi level, symbol c.b. represents the bottom of the conduction band and v.b. the top of the valence band. Symbols n, i and p represent n-type, intrinsic and p-type semiconductors, respectively.
In contrast, in SQW or MQW semiconductor lasers, one or a large number of hetero structures of GaAs and AlGaAs, for example, are superposed to form a potential well(s) and to reduce the freedom of an electron wave in a current flowing direction. In this manner a high recombination ratio is obtained by localizing the electrons, the holes and the excitons in at least one-dimensional direction. FIG. 2 shows the energy diagram of such a laser. This drawing conceptually shows the energy diagram in the same way as in FIG. 1. The portion that corresponds to 13 in FIG. 1 is MQW formed by alternately superposing AlGaAs 232 and GaAs 231. It is known theoretically that in the case of an exciton equivalent to a hydrogen atom, for example, the spread of a two-dimensional exciton is 1/2 of that of the wave function of the three-dimensional exciton.
In FIG. 2, reference numeral 20 represents the Fermi level, and 22 and 24 represent barrier layers of n-type and p-type, respectively.
In accordance with the heretofore known semiconductor laser technique, a high recombination ratio is obtained by forming SQW or MQW to reduce a threshold value and to obtain high efficiency. (Refer, for example, to "A Prospective in Superlattice Development", Physical Society of Japan, 1984, by Leo Esaki.) Nonetheless, sufficiently high efficiency cannot be obtained and semiconductor lasers having higher efficiency have therefore been desired.